Single-pass and sorbent dialysis systems both provide treatment for patients with acute or chronic kidney disease. Both systems deliver dialysate to the dialyzer in prescribed amounts to cleanse the blood of impurities, correct the patient's body chemistry, and remove excess fluid. Sorbent dialysis differs from traditional single-pass dialysis in that sorbent systems use less water than single-pass machines and do not require special plumbing. Single-pass systems use approximately 120 liters of water during a typical 4-hour treatment. In single-pass dialysis, a water treatment system is required to continuously pump purified water into the system to be blended with the bicarbonate and acid bath to create the final dialysate. This requires special plumbing to connect the single-pass machine to both the water treatment system and to a drain into which the used dialysate and rejected source water are disposed.
By utilizing sorbent technology, a dialysis system can provide highly-pure dialysate for 3- to 8-hour treatments using only 6 liters of potable tap water. The sorbent cartridge purifies the initial dialysate and continuously recirculates and regenerates the dialysate throughout the treatment. This not only eliminates the need to purchase and maintain an expensive water treatment system, but provides a high degree of transportability compared to conventional dialysis systems. Because sorbent systems do not require special wiring or plumbing, sorbent dialysis can be performed almost anywhere: in dialysis centers, hospital rooms, nursing homes, and home-care environments.
Sorbent systems provide a gentle way to achieve an electrolyte and chemical balance. Single-pass machines deliver a constant dialysate prescription to the patient. This forces the patient's body chemistry to change to match the dialysate prescription. This can cause some of the common side effects often associated with single-pass dialysis, such as nausea, cramping, and hypotension. During a sorbent dialysis treatment, urea is dismantled within the cartridge and combined with other solutes to replenish the sodium chloride and sodium bicarbonate required to correct the patient's body chemistry. Because the patient's body fluid volume is much larger than the dialysate volume, the patient is able to control the dialysate. The sorbent cartridge performs multiple tasks: it serves as a dialysate purification system, maintains dialysate pH balance, and binds uremic wastes.
Six liters of potable tap water and prescribed amounts of sodium chloride, sodium bicarbonate, and dextrose are used to create the initial dialysate solution. This mixture is then passed through the sorbent cartridge. As it flows through the cartridge, bacteria, pyrogens, endotoxins, metals, and organic solutes are removed from the initial dialysate. The purified dialysate is stored in the dialysate reservoir bag until it is circulated to the dialyzer. Once it leaves the dialyzer, the spent dialysate and the patient's ultrafiltrate fluid pass through the sorbent cartridge, where both are converted into partially regenerated dialysate, known as cartridge effluent. An infusate system adds calcium, carbon dioxide, magnesium, and potassium to form a fully regenerated dialysate, which then flows back into the dialysate reservoir bag, ready to be sent to the dialyzer.
Zirconium phosphate (ZrP) particles and hydrous zirconium oxide (HZO) particles are used as ion-exchange materials and are particularly useful as a sorbent material in regenerative kidney dialysis. Zirconium phosphate in the sodium or hydrogen form serves as a cation exchanger and absorbs cations such as ammonium (NH4+), calcium (Ca2+), potassium (K+), and magnesium (Mg2+). In exchange for absorbing these cations, ZrP releases two other cations, sodium (Na+) and hydrogen (H+). Hydrous zirconium oxide in the acetate form acts as an anion exchanger. Thus, it binds anions such as phosphate (P−) and fluoride (F−) and releases acetate (CH3COO−) in exchange. Hydrous zirconium oxide is also an excellent adsorbent for metals, such as iron, mercury, lead, and aluminum.
The sorbent cartridge containing ZrP and HZO ion-exchange materials has been historically used for the REDY (REgenerative DialYsis) system. The REDY sorbent cartridge consists of several layers through which used dialysate passes: i) a purification layer consisting of activated charcoal; ii) an enzyme layer consisting of urease; iii) a cation exchange layer consisting of ZrP; iv) an anion exchange layer consisting of HZO; and v) an adsorbent layer consisting again of activated carbon. During regenerative dialysis, the used dialysate moves up through the layers of the cartridge. The enzymatic urease converts urea into ammonium carbonate. The ammonia and ammonium ions are then removed by the zirconium phosphate in exchange for H+ and Na+ ions. The carbonate from the urea hydrolysis then combines with H+ to form bicarbonate (HCO3−) and carbonic acid (H2CO3). Carbonic acid is an unstable organic acid; most of it quickly breaks down into water and carbon dioxide molecules (CO2). The HZO (containing acetate as a counter ion) removes HCO3−, P−, and other anions (e.g., F− in water), and releases acetate. The activated carbon absorbs organic metabolites such as creatine, uric acid, and nitrogenous metabolic waste of the patient as well as chlorine and chloramines from the water. The CO2 gas bubbles are vented from the cartridge.
The safety and efficacy record of the REDY system has been well established. Nevertheless, the REDY cartridge can produce a variation of dialysate composition and pH during the treatment with the production of bicarbonate and carbonic acid, and the continuous release of Na+ by the cartridge.
Current zirconium phosphate (ZrP) based dialysis applications, such as the REDY cartridges, contain a large amount of lattice H+ ions even when it is titrated to a pH range of 5.75-6.45. During sorbent dialysis, these lattice H+ ions of ZrP will react with the NaHCO3 in dialysate causing initial decomposition of bicarbonate to CO2 gas and adsorption of Na+. After depletion of H+ ions and loading up of Na+ in ZrP, progressively, the NH4 adsorption mechanism will then switch to ion-exchange with adsorbed Na+ in ZrP. This will cause increasing release of Na+, accompanied by a rise of HCO3− level, and formation of CO32− from urea hydrolysis. Consequently, the use of ZrP alone for sorbent dialysis can cause a variation in Na+, HCO3−, and pH in regenerated dialysate during treatment.
The Na+ and bicarbonate level in the dialysate can also vary depending on the blood urea nitrogen (BUN) level of the patient. Thus, the REDY dialysis therapy has to provide several dialysate prescriptions to balance the pH and the Na+ level in the patient for the correction of hyper and hyponatremia. Also a conductivity alarm system is generally present to keep the Na+ level in the dialysate below a safe limit.
A need exists for ion-exchange materials for sorbent dialysis systems that can maintain steady and predictable dialysate compositions. Sorbent cartridges containing such materials could regenerate spent dialysate to normal and balanced Na+, HCO3− and pH levels, and without formation of CO2 gas bubbles. A need also exists for ion-exchange materials and sorbent dialysis systems that can remove toxic metal and non-metal ions from tap water in preparation of purified dialysate for dialysis.